I want to start with I love snorkeling and have many times over the years from everything to rivers with big alligator gar, sounds with manatees, and various tropical places like Hawaii and the Bahamas.
First, I just want to talk about the excursion you're talking about. At Pearl Island (I've never been but I have been researching for my next cruise) generally I've seen they give you a life jacket you can put around your waist and you go out and snorkel. The staff there are feeding the fish drawing them in so usually there is TONS of fish to see. There is some really good videos on YouTube where you can see people snorkeling there.
Snorkeling can be a struggle when you're on a "tour" like that because you need to keep up with the pace. So for someone like me its no problem but on one of my last cruises I took my 8 year old son (strong swimmer) and my 11 year old nephew (decent swimmer) out in Cozumel. We all had life jackets but it was a disaster. My son did perfectly fine to the point I had to try to slow him down but my nephew really did not have a great time. He seemed to panic and worry about everything which meant 95% of the time he was bobbing around in the water with his head out not looking at anything. Keep in mind, my nephew is all about nature, fish, outdoorsy stuff, etc. I think the part that really got him is that we were on a tour so it made him feel rushed which made the whole situation worst.
He is coming on our November cruise and we've opted to snorkel in Castaway Key instead. They don't feed the fish (at least not that I've seen) so there isn't as many but you see more acting naturally in the area. Plus some of the "reef" is Disney items! Its at your own pace so it'll be a much better experience for him (and some younger siblings that are coming and will do it for the first time) -- Also I wanted to add, with all this happening in Cozumel I was working with him the whole time which meant I also didn't get to really enjoy the experience.
So for a first time I'd really consider doing it at castaway key.